The invention relates to a transmission with torque division in particular for a helicopter rotor drive.
Such transmissions have the advantage that the torque to be transmitted is distributed among several gear engagements acting in parallel so that the gear forces per gear engagement are reduced and a higher power density can be achieved than in transmissions having no torque division.
In transmissions without torque division, increasing the transmissible torque by larger tooth width is possible only to a limited extent. In spur gears, there appears the danger of edge wear caused by housing deformation or imprecision in the production. In broad bevel gears or crown gears, problems appear with the tooth shape and it is possible above a certain tooth width to produce spiral bevel gears only at considerable expense or not at all.
U.S. Pat. No. 5,135,442, regarded as the closest art, has disclosed an angle transmission in which the power of a driving pinion is divided between two bevel gears lying opposite to each other on both sides of the pinion. Each one of the bevel gears is non-rotatably connected, via a shaft, with one other pinion. The added pinions are in simultaneous tooth engagement with a summarizing gear. In this solution, the large number of parts is a disadvantage.
For higher one-step ratios in the divided stage, a larger axial distance of the shafts and, accordingly, larger installation space would be needed.
The problem on which the invention is based is to provide a transmission with torque division which allows a high ratio and high power density in small installation space with light weight and a small number of functional elements.
This problem is solved, according to the invention, by the fact that a first power transmission branch has, as known per se, a first rotatable driving pinion which is in constant engagement with a first toothed wheel on the output side and at least one other power transmission branch has one other driving pinion co-axial to the first driving pinion which is in constant engagement with one other toothed wheel on the output side co-axial to the first toothed wheel on the output side.
The co-axial arrangement is very favorable in relation to number of parts and required space.
In an advantageous development of the invention, a torsionally elastic member is disposed in the torque flow of each of the power transmission branches. Hereby the power or torque distribution to the two power transmission branches can be definitely controlled and there is achieved an insensitivity to the deformations which can be produced in driving operation due, e.g. to static or thermal loads.
When the ratio of the torsional remittances of the torsionally elastic members is substantially corresponding to the (rolling) radial ratio of the associated driving pinion (the same ratio applies to the toothed wheels on the output side), the magnitude of the tangential forces in the tooth engagements of the power transmission branches is substantially equal so that when the tooth widths are equal a uniform load of the teeth is obtained. In this connection, the expression torsional resistance is understood as measure for the torque with which the part concerned counteracts a torsion around a certain angle.
A compact system is obtained when the torsionally elastic members between a branching point on the input side and the driving pinions are in co-axial pinion shifts disposed within each other with a torsionally resilient design, it is advantageous that the driving pinion of the radially outer pinion shaft be torsionally supported on the radially inner pinion shaft.
On the branching point, the pinion shafts are preferably non-rotatably interconnected by a positive fit spline, e.g. a toothed shaft spline. In an advantageous development, torsion resilient sections lie between the branching point and the pinion teeth so that the pinions are rotatable against each other under load around a load-dependent angle.
A very simple, high-load and reliable rotatable support of the driving pinion of the radially outer pinion shaft upon the radially inner pinion shaft is obtained when the support is designed as sliding bearing, the bearing races being integrated in the driving pinion or the pinion shaft. The supporting properties can be advantageously affected by a non-ferrous metal coating of the bearing races.
A thin-walled hollow shaft section is an advantageous development of a torsion-resilient shaft section in which the tensions are minimal compared to alternative developments. Especially in the radially outer pinion shaft, the wall thickness must be selected small to obtain torsional resilience. The polar surface inertia torque of the cross-section of the radially inner shaft is anyway substantially smaller due to the great dependence on the radius.
An alternative development of a torsion-resilient shaft section has, e.g. longitudinally oriented recesses. In other cases, it can be advantageous to produce the outer pinion shaft of a material of low thrust module, such as titanium.
To adjust the load portions of the power transmission branches, it is advantageous that the relative rotational position of the pinion shafts be adjustable at least once. The adjustability can be achieved, for instance, by providing on the branching point an helical-cut spline and the axial position of both pinion shafts being adjustable by means of spacers. But adhesive joints or frictionally coupled, pressed connections on the branching point or between pinion shaft and pinion, likewise, are possible so that the rotational position can be adjusted once during the assembly.
A development where at least one of the driving pinions is designed forming a single piece with the pinion shaft is advantageous with regard to the number of parts.
The inventive transmission can be used in several advantageous constellations.
An embodiment in which the toothed wheels on the output side are designed as cylindrical spur gears is advantageously suited to a parallel arrangement of input and output of this transmission stage.
When the toothed wheels on the output side are designed as bevel gears, the most diverse angles can he obtained between input and output of the transmission.
When the toothed wheels on the output side are designed as crown gears and the cylindrical driving pinions are disposed forming a right angle therewith, axial movements of the driving pinions do not act upon the radial teeth play.
With driving pinions designed as bevel gears, it is possible to adjust the teeth play by setting the axial position, e.g. by means of spacers. Combined with toothed wheels on the output side likewise designed beveledxe2x80x94as describedxe2x80x94the most diverse angles can be implemented between input and output of the transmission, the extensions of the axles normally having a common intersection point.
Desirable in other installation cases is a center distance which is made possible when the driving pinion and the toothed wheels on the output side form hypoid pairs.
The known advantages of helical-cut teeth or spiral teeth are a higher degree of contact and less teeth noises. By an axial displacement of an helical-cut pinion, along its axis of rotation, it is favorably possible to affect the load portion of the power transmission branch concerned.
It is advantageous that the helix angle of the wheel pairs of the power transmission branches have opposite directions. In this manner, a compensation of axial forces takes place so that a smaller axial bearing is sufficient. If the pinion shafts are fixed axially to each other, but supported as a unit with axial play against the toothed wheels on the output side, there appears a power-distribution effect dependent on the helix angles.
Regarding an easy assembly, it is advantageous that the helix angles of the wheal pairs of the power transmission branches have the same sense of direction.
For kinematic reasons, it is necessary that the ratios of the power transmission branches be exactly alike. Especially simple assembly conditions are obtained when the driving pinions and the toothed wheels on the output side of the power transmission branches each have the same number of teeth. When the different power transmission branches have different rolling radii, it is advantageous that the ratio of the modules of the teeth substantially corresponds to the (rolling) radii ratio. Favorable teeth shapes are possible in this manner.
For higher total ratios, it is favorable that a transition stage be provided on the output side.
Finally, it is advantageous that one pinion of one power take-off unit be constantly engaged with one of the toothed wheels on the output side. The pinion creates a permanent operative connection of the tail rotor drive with the main rotor.